1. Field of the Invention
The present invention relates to an optical modulator used in an optical communication system, wherein data signals are converted from electric signals into optical signals by controlling optical phase with electro-optical effect.
2. Description of the Related Art
In recent years, progress is being made in various fields regarding optical communication systems to realize increased speed, increased capacity, and reduction in size. With optical transmission devices, optical waveguide type external modulators using electro-optical crystals such as lithium neonate (LiNbO3, hereafter may be written simply as “LN”) substrates and the like instead of performing direct modulation with a laser diode, from the perspective of broadband properties, chirp properties, and so forth.
With such modulators, a Mach-Zehnder type topical waveguide is formed on an electro-optical crystal substrate formed of LN or the like, by forming a metal film of titanium (Ti) or the like on a part of a crystal substrate of LN or the like and then effecting thermal dispersion thereof, or forming the metal film and effecting proton exchange in benzoic acid. A buffer layer of silicon dioxide (SiO2) is formed on the substrate wherein the optical waveguide has been formed. Further, in the case of a substrate cut parallel to the Z-axis of the crystal axis direction of the LN substrate, an LN modulator is fabricated by providing two signal electrodes (one positive signal electrode and one negative signal electrode), or a one signal electrode and one ground electrode, on a linear portion of a Mach-Zehnder optical waveguide. This buffer layer is provided for reducing absorption loss with regard to light propagated over the optical waveguide. An RF (Radio Frequency) signal source for generating modulation signals is connected to the signal electrode(s).
There are also known an optical modulator wherein an operation point for RF signals applied to the signal electrode, i.e., a bias electrode for applying bias voltage for controlling the DC bias component of the RF signals, is provided, with the bias electrode being provided on a linear portion other than the linear portion of the Mach-Zehnder optical waveguide where the signal electrode(s) is (are) provided. This optical modulator is disclosed in Japanese Unexamined Patent Application Publication No. 2003-233042.
There is also known an optical modulator wherein an X-cut substrate is used and a buffer layer having a width greater than the width of the signal electrode(s) is embedded in the surface portion of the X-cut substrate only underneath the signal electrode(s). This optical modulator is disclosed in Japanese Unexamined Patent Application Publication No. 2000-122016.
There is also known an optical modulator using a Z-cut substrate, wherein a polarization inversion region is formed on a part of the linear portion of a Mach-Zehnder optical waveguide, and RF signals of the same polarity are applied to signal electrodes at two linear portions of the Mach-Zehnder optical waveguide, thereby suppressing chirp. This optical modulator is disclosed in Japanese Unexamined Patent Application Publication No. 2003-202530.
With the buffer layer below the signals electrode(s) in the optical modulators described above, a thick buffer layer will result in a broader modulation bandwidth but higher modulation voltage, while a thin buffer layer will result in lower modulation voltage but a narrower modulation bandwidth. However, the bias voltage applied to the optical waveguide beneath the bias electrode across the buffer layer is not dependent on the thickness of the buffer layer.